Aqueous solutions of chlorine dioxide, because of the high oxidation power of chlorine dioxide, are frequently used in water-treatment methods. The area of application ranges from the disinfection of drinking water and bathing water to treatment of service water and wastewater. In comparison with the classic oxidizing disinfectants chlorine and hypochlorite, chlorine dioxide is distinguished in use by an outstanding ecobalance. Thus, when chlorine dioxide is used, only small amounts of AOX ("adsorbable organic halogen compounds"; overall parameter for all chlorine, bromine and iodine compounds which can be adsorbed to activated carbon and have very different hazard potentials) and virtually no trihalomethane (TEM, haloforms) are formed.
It is known, for example from DE-C 843 999, for the preparation of chlorine dioxide, to proceed from a chlorite, for example sodium chlorite and to oxidize this to chlorine dioxide with an oxidizing agent, for example sodium peroxodisulphate, in aqueous solution according to the summation equation EQU 2NaClO.sub.2 +Na.sub.2 S.sub.2 O.sub.8.fwdarw.2ClO.sub.2 +2Na.sub.2 SO.sub.4.
To obtain a reaction rate as high as possible, the reaction solution in this known process is set to a pH of 5-9, if appropriate using a buffer, contains the oxidizing agent in a stoichiometric excess and can be heated to up to 65.degree. C. for further acceleration of the reaction. The chlorine dioxide forming is continuously expelled from the reaction solution by passing in inert gas and is collected in an absorption tower.
Although this known process gives a chlorine dioxide of very high purity at good yields, based on the chlorite used, it is highly restricted in its applicability and is little suited to industrial application "on site", not only because of the high cost of equipment for separating the chlorine dioxide, but also because of the high explosion hazard of the gaseous chlorine dioxide. Furthermore, for many applications, for example for drinking-water treatment, there is also no possibility of avoiding the separation of the chlorine dioxide and using the reacted reaction solution as such for the disinfection, since this solution still contains too much residual chlorite and, moreover, is contaminated by toxic chlorate formed as by-product.